The present invention relates to hand tools, and pertains particularly to an improved impact tool.
The hand held and manipulated hammer is perhaps one of the oldest, most widely used, and most powerful tool known to man. Hammers can produce very large forces by making use of the sudden change of momentum upon impact: the change of momentum is equal to the force integrated over time, since the time of impact is infinitesimally small, the force can be very large. Momentum is at the foundation of the hammer's usefulness. Most hammers that are presently in common use have heavy impact members (hammerheads) mounted at the end of long handles to provide the leverage which gives the hammerhead speed and momentum. However, motion of the hammerhead at the end of a long handle is difficult to control, and so much power in unskilled hands becomes dangerous. Many fingers have been injured while performing jobs as simple as driving a nail.
A guide directed hammer has recently been invented. This invention is described in U.S. Pat. No. 4,662,557, it will be referred to herein as the "prior art patent". The apparatus has an impact member to deliver the momentum to a work piece. The impact member slides along a guide which confines the movement of the impact member to within a predetermined path leading to the work piece. Thus the risk of injuries due to poorly directed hammerhead is minimized. Input movement to the device is received through input means which includes a handle. In use, the handle is grasped by the user's hand and pushed forward while the forward end of the guide is pushed against the stationary surface to be nailed. A speed multiplying mechanism transforms the input movement to the output movement of the impact member, driving the impact member to move at several times the speed of the hand that pushes the device's handle. The speed multiplying mechanism thus provides the speed multiplying benefit of a long-handled hammer, but it does so without letting the impact member fly out of control.
The prior art patent describes several possible mechanisms for accomplishing the purpose of speed multiplication. These mechanisms include (1) cable and pulleys, (2) levers, (3) rack and pinion gears, and (4) hydraulic or pneumatic cylinders. Each one of these mechanisms has its unique advantages and peculiar difficulties in implementation.
One of the difficulties is the introduction of unwanted torques to the apparatus' moving parts. For example, in the case of the impact member during impact, a very large force is applied to the work piece, if this force had been applied along an axis that does not pass near the impact member's center of gravity, a very large torque will be generated to act on the impact member. Such a large torque can quickly destroy the impact member or the part of the guide around the impact position. It was realized in the prior art patent that the impact with the work piece must always produce a force along an axis that passed very near to the impact member's center of gravity. However, other smaller torques, which can have important effects on the apparatus' efficiency have been mostly ignored in the prior art:
While the impact member is accelerating towards the operative position (the position of impact), it is driven by a force from the speed multiplying mechanism. This driving force is the output force of the speed multiplying mechanism, it is applied along the output axis, and generates an output movement. If the output axis is not positioned along a line through the impact member's center of gravity, then a torque will be generated to act on the impact member.
In the case of the speed multiplying mechanism, an input force is applied along an input axis, and the mechanism delivers an output force along the output axis, if the input and output axes are not colinear, then a torque is generated to act on the speed multiplying mechanism. The embodiments shown in FIGS. 1-5 in the prior art patent all have assymetric speed multiplying mechanisms that are potential sources of unwanted torques.
Most of the torques generated by off-center forces are undesirable because they cause the moving parts to exert lateral forces on the support members. These lateral forces generate friction, causing the parts to "bind". The binding frictions are often self-regenerating, so once binding is started somewhere, it becomes progessly worse. And if the parts become deformed, then they bind even more. Therefore, whenever unwanted torques are present to the extend of threatening to bind, it becomes necessary to use very rigid parts to prevent deformation.
The binding effect of unwanted torques can be minimized by the use of precision parts and low friction bearings (such as ball bearings). However, the use of these parts make the apparatus costly. Such costs can be avioded by not having too much unwanted torque in the first place. And it is the purpose of the present invention to cut down on the unwanted torques on the moving parts.
Different moving parts of the impact apparatus are effected to different degree by unwanted torques. In general, the fastest-moving parts are affected most adversely by the unwanted torques. So torque on the impact member causes more trouble than an equal torque on the guide. In some situations, it may be acceptable (in fact sometimes even desirable) to increase torque the guide or on the handle in order to reduce torque on the impact member.